This invention relates to coal gasifiers, and more specifically, to a high performance, multi-stage, pressurized, airblown, entrained flow coal gasifier system and a method of operating the same.
The Government of the United States of America has rights in this invention pursuant to Contract No. DE-FC21-90MC26308.
Current governmental air pollution standards limiting the level of emissions from the stacks of fossil fuel-fired power generation equipment have created an urgent need for clean burning fuels. An obvious solution to this problem is to burn fuels which are low in particulate matter and sulfur content, and to do so in a manner that will minimize NO.sub.x emissions. However, such fuels are in relatively short supply and also are relatively more expensive.
One solution to the problem, particularly in the United States where the basic proven energy reserves are predominantly coal, is the gasification of coal to produce a fuel gas suitable for firing in a steam generator and similar devices, or for firing in a gas turbine, which is part of an integrated gasification combined cycle system (IGCC). Generally speaking, coal gasification involves the reaction of coal, at high temperatures, with a gas containing oxygen, and steam to produce a gas, containing principally carbon monoxide (CO) and hydrogen (H.sub.2), which is suitable for use as a fuel gas.
The processes, which to date have been employed for purposes of achieving coal gasification, can be conveniently divided into three categories:
1. Fixed bed gasification where lump coal is supported on a grate or by other means and the flow of gas and coal may be concurrent or countercurrent;
2. Fluidized bed gasification where crushed or fine coal is fluidized by the gasifying medium, giving an expanded fuel bed that can be visualized as boiling liquid; and
3. Suspension or entrainment gasification where fine coal is suspended in the gasifying medium such that the fine coal particles move with the gasifying medium either linearly or in a vortex pattern.
At one time fixed bed gasifiers were felt by many to be the most attractive from the standpoint of economics. In this regard, the economically desirable features of fixed bed gasifiers were deemed to encompass the following features: their inherent relatively high carbon conversion ratio, the fact that a low producer volume is required per unit of gas production, and the fact that a minimum amount of fuel preparation is required in connection with the use thereof.
By way of exemplification and not limitation, one example of a fixed bed gasifier is that which forms the subject matter of U.S. Pat. No. 3,920,417 entitled "Method of Gasifying Carbonaceous Material", which issued on Nov. 18, 1975 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 3,920,417, there is provided a method of producing a clean low BTU fuel gas by the reaction of a carbonaceous fuel with free oxygen and steam in a downdraft fixed bed gasifier. The material to be gasified is introduced at the upper end of the fixed bed gasifier where it is preheated, dried and devolatilized by a stream of hot gas from the lower zones of the fixed bed gasifier, which has been drawn upward and which is withdrawn from the fixed bed gasifier at the upper end thereof. This upward flow of gas also serves to maintain the ignition level of the carbonaceous fuel material at a predetermined level within the oxidation zone of the fixed bed gasifier. The portion of gas withdrawn from the upper end of the fixed bed gasifier, which contains the volatiles released by the carbonaceous material, is reintroduced into the fixed bed gasifier in the oxidation zone where the temperature is sufficiently high to effect thermal cracking of the volatiles to valuable hydrocarbon compounds. Continuing, as the carbonaceous fuel material moves downward through the shaft of the fixed bed gasifier it passes successively through oxidation and reduction zones where air or some other free oxygen containing gas, and steam, respectively, are introduced into the fixed bed gasifier. The resulting reactions convert the carbonaceous fuel material and other reactants to an incandescent char matrix extending to the shaft of the fixed bed gasifier, gaseous products (the makeup thereof depending upon the level of the fixed bed gasifier), and a molten slag.
A subsequent modification in the design of fixed bed gasifiers wherein a second stage has been added in cooperative association with the fixed bed gasifier forms the subject matter of U.S. Pat. No. 4,069,024 entitled "Two-Stage Gasification System", which issued on Jan. 17, 1978 and which is assigned to the same assignee as the present application. In accordance with the teachings of U.S. Pat. No. 4,069,024, there is provided a method and apparatus for directing a low BTU content synthesis gas, as produced in a fixed bed gasifier, for example, of the type previously disclosed in U.S. Pat. No. 3,920,417, through a second stage having a serially interconnected pyrolyzer of so-called "spouting bed" design wherein the hot synthesis gas from the fixed bed gasifier is contacted by a hot carbonaceous charge and lime to utilize its high sensible heat to produce a cool, higher BTU product. Reactions take place between some of the carbon of the charge and hydrogen of the gas to increase the hydrocarbon content of the gas before it is conducted to its place of use, while the lime reacts with the sulphur to reduce if not eliminate the sulphur content of the gas. The remaining charge of char is devolatilized and passed on to the fixed bed gasifier where a hot oxidizer and steam are added thereto to produce more synthesis gas that is supplied back to the second stage, i.e., to the pyrolyzer, for reaction with the carbonaceous charge and lime.
Turning next to a consideration of entrainment, i.e., suspension, gasification, there are examples thereof to be found in the prior art. By way of exemplification and not limitation in this regard, one such example of an entrainment gasification process is that which forms the subject matter of U.S. Pat. No. 4,158,552 entitled "Entrained Flow Coal Gasifier", which issued on Jun. 19, 1979 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,158,552, a high temperature level of product gas stream is formed by burning primarily char with the existing air supply. Immediately thereafter additional char is introduced into the high temperature stream for gasification of these carbon particles. Then, following the endothermic gasification reaction which cools the gases, new fresh coal is introduced with this coal being devolatilized at relatively low temperature, thus utilizing low temperature heat. Entrained char particles are thereafter removed from the gas stream and reintroduced into the gasifier. The low temperature devolatilization of the fresh coal is achieved by gas temperatures at a level which is insufficient to effectively continue the carbon gasification process. Accordingly, more of the available heat is used for the basic purpose of the coal gasification operation, which is of course to produce gas having the maximum reasonable heating value.
Another example of an entrainment gasification process is that which forms the subject matter of U.S. Pat. No. 4,343,627 entitled "Method of Operating a Two-Stage Coal Gasifier", which issued on Aug. 10, 1982 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,343,627, an object of the invention disclosed therein is to increase the effectiveness of an entrained flow gasifier by enhancing its ability to produce a product gas having an increased heating value. Moreover, the manner in which this is accomplished is that steam is used as the conveying medium for carrying the additional coal to be injected into the reduction zone from the coal source to the reduction zone. Upon entering the reduction zone, the steam reacts with the additional coal to form carbon monoxide and hydrogen. Therefore, rather than lowering the heating value of the product gas as is the case when either air or inert gas are used as the conveying medium, the steam reacts to form additional carbon monoxide and hydrogen thereby increasing the heating value of the product gas formed in the reduction zone. Further, by using steam as the conveying medium, the possibility of an explosion in the transport line between the coal source and the reduction zone is eliminated.
Yet another example of an entrained gasification process is that which forms the subject matter of U.S. Pat. No. 4,610,697 entitled "Coal Gasification System with Product Gas Recycle to Pressure Containment Chamber", which issued on Sep. 9, 1986 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,610,697, there is provided a pressurized coal gasifier for producing a clean, particulate free fuel gas suitable for use in a gas turbine-generator or a feedstock for a methanation or other chemical process. The subject gasification system includes a pressure containment chamber surrounding an inner, water-cooled chamber wherein a gasification reaction takes place, a second pressure containment chamber surrounding a process gas heat exchanger for cooling the gas produced by the gasification reaction, and a conduit between the first and second vessels having an outer pressure containing wall and an inner water-cooled transfer duct for conducting the produced gas from the gasifier vessel to the heat exchanger. Continuing, the gasifier vessel includes a slag tap disposed at the bottom for removal of any molten slag produced by the gasification reaction. Differential thermal expansion between the pressure containment chamber and the water-cooled gasifier chamber is accommodated without loss of inter-chamber sealing by a water seal between the chambers without the occurrence of gas leakage into the annular volume.
Still another example of an entrained gasification process is that which forms the subject matter of U.S. Pat. No. 4,680,035 entitled "Two Stage Slagging Gasifier", which issued on Jul. 14, 1987 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,680,035, a two stage coal gasifier is provided that has a vertically elongated gasifier chamber for upward flow of the gas to an opening at the top. Tubes comprising the walls of the chamber form a centrally located slag tap opening at the bottom of the chamber. In the lower portion of the gasifier is a combustion section where fuel injection nozzles inject coal tangentially along with combustion supporting air to supply the heat source. At an upper elevation is a reductor section wherein additional fuel is introduced for the purpose of gasifying this additional fuel. Between the combustor section and the reductor section there is a centrally located baffle. This baffle is larger in plan area than the tap opening and is arranged so that any slag falling around the baffle will land some distance remote from the slag tap opening. The centrally located baffle minimizes the radiation loss from the combustor to the reductor, blocks the central portion of the gasifier chamber so that gas is not drawn back down from the reductor section, and forces the slag falling from the reductor to enter the combustor at a location outboard of the slag tap opening so that it has time to be heated substantially before passing to the slag tap opening.
Although gasifier systems constructed in accordance with the teachings of the various U.S. patents to which reference has been had hereinbefore have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for such gasifier systems to be further improved if increased use thereof is to be realized. To this end, there has been evidenced in the prior art a need for a new and improved gasifier system that would enable higher coal-to-gas efficiencies to be attained through the use thereof than those known to be attainable through the use of currently available forms of gasifier systems. A need has thus been evidenced in the prior art for a new and improved gasifier system that would be capable of outperforming, in terms of cold gas efficiency, hot gas efficiency and carbon conversion, presently existing gasifier systems, be they of the fixed-bed type, of the fluidized-bed type, or of the entrained type. More specifically, there has been evidenced in the prior art a need for a new and improved gasifier system, which is capable of achieving a cold gas efficiency of 85%, a hot gas efficiency of 95% and a carbon conversion of 99.99+%, which is to be contrasted with the performance that is capable of being achieved with current technologies wherein the cold gas efficiency, the hot gas efficiency and the carbon conversion for presently existing forms of fixed-bed gasifier systems are said to be 80%, 85% and 95-98%, respectively, and wherein the cold gas efficiency, the hot gas efficiency and the carbon conversion for presently existing forms of fluid-bed gasifier systems are said to be 72%, 80% and 93-98%, respectively, and wherein the cold gas efficiency, the hot gas efficiency and the carbon conversion for presently existing forms of entrained gasifier systems are said to be 67-70%, 75% and 99.99+%, respectively.
Moreover, there has been evidenced in the prior art a need for such a new and improved gasifier system that would be particularly characterized in a number of respects. In this regard, one such characteristic which such a new and improved gasifier system would desirably possess is that it be of the entrained flow type. Another characteristic which such a new and improved gasifier system would desirably possess is that it embody multiple stages. A third such characteristic which such a new and improved gasifier system would desirably possess is that it be pressurized. A fourth such characteristic which such a new and improved gasifier system would desirably possess is that steam be employed for the transport of char. A fifth such characteristic which such a new and improved gasifier system would desirably possess is that steam also be employed for the transport of coal. A sixth such characteristic which such a new and improved gasifier system would desirably possess is that for purposes of increasing the rate at which gasification reactions take place therewithin as well as for purposes of enhancing slagging the temperature within the first stage would optimally be on the order of 3500.degree. F. A seventh such characteristic which such a new and improved gasifier system would desirably possess is that, if so desired, limestone/dolomite is capable of being employed therewith for purposes of effecting sulfur removal.
To thus summarize, a need has been evidenced in the prior art for such a new and improved gasifier system that would enable higher coal-to-gas efficiencies to be attained therewith and that in doing so would render it possible therewith to outperform prior art forms of gasifier systems in terms of the cold gas efficiency, the hot gas efficiency and the carbon conversion achievable therewith. Moreover, such results would be accomplished with a new and improved gasifier system embodying a first stage within which char combustion would take place at an optimum temperature of 3500.degree. F. so as to thereby both increase the rate at which gasification reactions take place therewithin and to enhance slagging and wherein the char that is combusted within the first stage is transported thereto by means of steam. In addition, such results would be accomplished with a new and improved gasifier system embodying a second stage within which the gasification of the char would take place and wherein the char that is gasified within the second stage is transported thereto by steam. Furthermore, such results would be accomplished with a new and improved gasifier system embodying a third stage within which the coal is devolatilized and wherein the coal that is devolatilized in the third stage is transported thereto by means of steam also. Last but not least, such results would be accomplished with a new and improved gasifier system embodying heat exchanger means whereby the gas in which the char resulting from the devolatilization of the coal is entrained upon leaving the third stage would pass through the heat exchanger means whereby the steam would be produced that would be employed to transport the char to the first stage and the second stage thereof before the char-entrained gas would pass through the separator means within which the char is separated from the gas for transport by means of the aforementioned steam to the first stage and the second stage thereof.
It is, therefore, an object of the present invention to provide a new and improved system for gasifying coal to produce therefrom a fuel gas suitable for firing in a gas turbine, which is part of an integrated gasification combined cycle system (IGCC), or for firing in a steam generator and similar devices.
It is a further object of the present invention to provide such a gasifier system which is of the entrained flow type.
It is another object of the present invention to provide such an entrained flow gasifier system which is pressurized.
It is still another object of the present invention to provide such a pressurized, entrained flow gasifier system which embodies multiple stages.
Another object of the present invention is to provide such a multi-stage, pressurized, entrained flow gasifier system which embodies a first stage within which char combustion takes place at an optimum temperature of 3500.degree. F. so as to thereby increase the rate at which gasification reactions take place therewithin as well as to enhance slagging.
A still another object of the present invention is to provide such a multi-stage, pressurized, entrained flow gasifier system which embodies a second stage within which the gasification of the char takes place.
A further object of the present invention is to provide such a multi-stage, pressurized, entrained flow gasifier system which embodies a third stage within which the coal is devolatilized.
A still further object of the present invention is to provide such a multi-stage, pressurized, entrained flow gasifier system which is characterized in that steam is employed to transport the char to the first stage thereof as well as to the second stage thereof.
Yet an object of the present invention is to provide such a multi-stage, pressurized, entrained flow gasifier system which embodies heat exchanger means through which the gas in which the char is entrained is made to pass upon leaving the third stage such that steam is thereby produced which in turn is employed to thereby effectuate the transport of the char to the first stage and the second stage thereof.
Yet a further object of the present invention is to provide such a multi-stage, pressurized, entrained flow gasifier system which is characterized in that steam also is employed to transport the coal to the third stage thereof.
Yet another object of the present invention is to provide such a multi-stage, pressurized, airblown, entrained flow gasifier system which is characterized by its high performance wherein higher coal-to-gas efficiencies are capable of being attained therewith than those capable of being attained with previously existing forms of gasifier systems.
Yet still another object of the present invention is to provide such a high performance, multi-stage, pressurized, airblown, entrained flow gasifier system which is characterized in its ability to outperform previously existing forms of gasifier systems in terms of cold gas efficiency, hot gas efficiency and carbon conversion.